As skid control systems for aircraft have become progressively faster and more sophisticated, they have evolved toward the use of digital computing techniques for solving certain required equations. In view of the many variables experienced in terms of surface condition, weather conditions, etc., and in view of the fast computing capabilities available, it becomes more and more important that the wheel or tire speed-responsive means have good resolution and fast response time. Prior art systems have relied upon wheel speed signal generators which are really wheel-driven alternators having a substantial number of poles so that an alternating current signal is generated whose instantaneous frequency is proportional to instantaneous speed of the wheel. Since wheel speed generators should be as small and light as possible, there is, of course, a practical limit as to the number of poles, hence cycles per wheel revolution, which can be built into such an alternator.
The braked wheel can, however, change its operating characteristics from an acceptable deceleration to an unacceptable skid in a relatively few degrees of rotation, which may mean that even with as many as 128 poles, the number of signal cycles available at the time of a skid to produce a meaningful input signal may be so few that the associated control system may not respond quickly. During this time the skidding may well either continue or get worse before any correction can take place. Thus the wheel speed sensor has come to be recognized as imposing a significant limiting factor preventing substantial improvement in the performance of skid controls.